This article is on the P Block Elements Class 12 Notes of Chemistry. The notes on P Block Elements of class 12 chemistry have been prepared with great care keeping in mind the effectiveness of it for the students. This article provides the revision notes of the P Block Elements chapter of Class 12 for the students so that they can give a quick glance of the chapter.
This chapter of Class 12 has been divided into four articles. This article (Part 1) includes the Group 15 elements. The second article (Part 2) is on Group 16 elements. The third article (Part 3) is on Group 17 elements. At last, the fourth article (Part 4) is on Group 18 elements.
P Block Elements (Part 1)
p-block elements are placed in group 13 to 18 of the periodic table. Their valence shell electronic configuration is ns2np1-6 (except He which has 1s2 configuration).
The first member of each of the groups 13-17 of the p-block elements differs in many aspects from the rest of the members of their respective groups. This anomalous behaviour is due to small size, high electronegativity and inability to expand its octet due to the absence of d-orbitals in the valence shell.
Group 15 Elements
The elements of Group 15 are as follows: nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb) and bismuth (Bi). Bi is a typical metal, N and P are non-metals, and As and Sb are metalloids. The valence shell electronic configuration of this element is ns2np3.
Molecular nitrogen comprises about 78% by volume of the earth’s atmosphere. It occurs as sodium nitrate, NaNO3 (Chile saltpetre) and potassium nitrate, KNO3 (Indian saltpetre) in earth’s crust. Nitrogen is also an important constituent of amino acids, proteins and nucleic acids in plants and animals.
Major amounts of phosphorus occur in a mineral family known as apatites of the general formula, 3Ca3(PO4 )2.CaX2, Ca9 (PO4)6.CaX2 or Ca10 (PO4 )6 X2 (where X = F, Cl or OH) which are major components of phosphate rocks.
Arsenic, antimony and bismuth are found mainly as sulphide minerals, e.g., arsenopyrite (FeAsS), stibnite (Sb2S3), bismuth glance (Bi2S3).
The valence shell electronic configuration of these elements is ns2 np3 (n = number of shells) i.e., s-orbital is fully filled while p-orbital is half-filled in order to make it extra stable.
Atomic and Ionic Radii:
On moving down the group as the atomic number increases, the atomic radii increases due to increase in the number of shells.
Ionization enthalpy of group 15 elements is much more than that of group 14 elements in the respective period due to smaller atomic radii and extra stable electronic configuration of the half-filled orbital. On moving down the group, ionization enthalpy decreases from N to Bi due to the gradual increase in atomic radii and shielding effect.
On moving down the group from N to Bi, electronegativity decreases due to the gradual increase in atomic size. Electronegativity of N is the 3rd highest in the periodic table.
Different physical properties of group 15 elements are given below
(i) All the elements of this group are polyatomic. Nitrogen is a diatomic gas while other members are solids.
(ii) Metallic character increases down the group due to increase in atomic size and the decrease in ionisation enthalpy.
(iii) On moving down the group, the melting point first increases from N to As (because of the increase in atomic size) and then decreases to Sb and Bi (due to inert pair effect).
(iv) Boiling points, in general, increase from top to bottom.
(v) All elements tend to show allotropy except nitrogen.
Chemical properties of group 15 elements are
Oxidation States and Trends in Chemical Reactivity:
The common oxidation states of group 15 elements are + 3, — 3 and + 5. The tendency to show -3 oxidation state decreases down the group because the atomic size and metallic character increases. Bi does not form any compound in — 3 oxidation state.
Stability of + 3 oxidation state also increases down the group due to inert pair effect. The stability of +5 oxidation state decreases down the group. Bi forms only one compound in + 5 state (BiF5).
Nitrogen when reacts with oxygen shows + 1, + 2, + 4 oxidation states. In some oxoacids, Phosphorus also shows + 1 and + 4 oxidation states. All oxidation states in the case of nitrogen, from +1 to +4 tend to disproportionate in acid solution.
Reactivity towards Oxygen:
The elements of group 15 form two types of oxides: E203 and E205. The oxide in the higher oxidation state is more acidic than that of lower oxidation state. Their acidic character decreases down the group. E203 type oxides of N and P are purely acidic while oxides of As and Sb are amphoteric and oxides of Bi are basic.
Reactivity towards Halogens:
These elements directly combine with halogen to form trihalides and pentahalides. All trihalides are stable (except NBr3 and NI3) and have a pyramidal shape. They are easily hydrolysed by water. In case of nitrogen, only NF3 is known to be stable. N does not form pentahalides due to the absence of d-orbital, however, P, As and Sb form pentahalides. Bi does not form pentahalides due to inert pair effect. Pentahalides involve sp3d hybridsation and have trigonal bipyramidal shape.
Reactivity towards Hydrogen:
These elements form volatile hydrides with the formula EH3 (where E = N, P, As, Sb, Bi). On moving down the group, thermal stability decreases from NH3 to BiH3 due to increase in the size and the bond length. Reducing character increases down the group due to the decrease in bond dissociation enthalpy.
The reducing character is as follows
NH3 <PH3 < AsH3 <SbH3 <BiH3
In case of nitrogen, all oxidation states from +1 to + 4 tend to disproportionate in acid solution and in case of phosphorus nearly all intermediate oxidation states disproportionate into + 5 and —3 in acid and in alkali solutions.
It is a diatomic gaseous molecule with a triple bond between the two atoms. Its bond enthalpy is very high. N forms pπ- pπ multiple bonds with itself and with other elements having a small size and high electronegativity (e.g., C, O). Other members are solids. They do not form pπ-pπ multiple bonds, they form single bonds as P – P, As-As, etc.
N—N single bond is weaker than P—P single bond due to the greater repulsion of the non-bonding electron is (due to small bond length). As a result, catenation tendency is weaker in nitrogen than phosphorus.
Dinitrogen is prepared in the laboratory by heating an aqueous solution of ammonium chloride with sodium nitrite.
NH4Cl(aq)+ NaNO2 (aq) —> N2 (g)+ 2H20 (l)+ NaCl (aq)
(NH4)2 Cr207 —∆—> N2 + 4 H2O + Cr2O3
Dinitrogen is rather inert at room temperature due to the high bond enthalpy of N—N triple bond.
It is used in the manufacture of ammonia and other nitrogen-containing chemicals. It is used to produce an inert atmosphere in industries and also in cryosurgery.
On small scale, NH3 is obtained from ammonium salts which decompose when treated with caustic soda or lime and on large scale it is manufactured by Haber’s process. NH3 acts as a Lewis base because it can donate one lone pair of electrons.
2NH4Cl+ Ca(OH 2 —> 2NH3 + 2H20+ CaCl2
(NH4)2SO4 + 2NaOH —> 2NH3 + 2H20+ Na2SO4
Haber’s process N2 (g) + 3H2(g) —> 2NH3 (g)
The optimum conditions for the production of NH 3 are
Pressure 200 x 105 pa (≈ 200 atm); Temperature —> 700 K
Catalyst -iron oxide with the small amount of K20 and Al203 .
Ammonia is used to produce nitrogenous fertilizers like ammonium nitrate, urea, ammonium phosphate and ammonium sulphate. Liquid ammonia is used as a refrigerant. It is also used in the manufacture of nitric acid.
Oxides of Nitrogen:
Nitrogen forms a number of oxides in different oxidation states. N20 and NO are neutral oxides, while other oxides of nitrogen are acidic. NO2 dimerises because it contains an odd number of valence electrons. The covalence of nitrogen in N205 is 4 because it has four shared pairs of electrons.
Nitric Acid (Oxoacids of Nitrogen):
Nitrogen forms oxoacids such as hyponitrous acid (H2N202), nitrous acid (HNO2) and nitric acid (HNO3). Nitric acid on large scale is prepared by Ostwald’s process in which Pt/Rh is used as a catalyst. This method is based upon catalytic oxidation of NH3 by atmospheric oxygen. It is a colourless liquid in its pure form and becomes yellow due to the presence of NO2. In the gaseous state, HNO3 exists as a planar molecule with the structure. It is a strong oxidising agent and attacks most metals except noble metals (Au and Pt).
4NH3 (g) + 5O2 (g) (from air) — Pt/Rh gauge catalyst /500 K, 9 bar— > 4 NO(g) + 6 H2O(g)
2NO (g) + O2(g) ↔ 2NO2(g)
3NO2 (g) + H2O (l) —> 2HNO3 (aq) + NO(g)
Brown Ring Test for Nitrates:
The familiar brown ring test for nitrates depends on the ability of Fe2+ to reduce nitrates to nitric oxide which on reaction with Fe2+ ions form a brown coloured complex. This test is usually carried out by adding freshly-prepared dilute ferrous sulphate to an aqueous solution containing nitrate ion, followed by careful addition of conc. H2SO4 along the sides of the test tube so that separate layer at the interface between the solution and sulphuric acid is formed. Brown ring thus formed indicates the presence of nitrate ion.
Nitric acid is used in the manufacture of ammonium nitrate for fertilizers, as an oxidiser in rocket fuels, in the pickling of stainless steel, etching of metals and also in the preparation of nitroglycerin.
Except for nitrogen, all the elements show allotropy. Phosphorus is available in many allotropic forms. White phosphorus is more reactive than red phosphorus due to angular strain in the P4 molecule. White phosphorus consists of the discrete tetrahedral P4 molecule while red phosphorus consists of polymeric chains of P4 tetrahedra.
Phosphine like ammonia is weakly basic in nature and gives phosphonium compounds with acids. Bond angle in PH4+ is higher than that in PH3 because lp-bp repulsion in PH3 reduces the bond angle to less than 109° 28′.
It is non-inflammable in its pure form and catches fire in contact with traces of oxidising agents like HNO3, Cl2 and Br2 vapours. It is water soluble and a colourless gas with a rotten fish smell and is highly poisonous. It is used in Holme’s signals, in smoke screens and for making metallic phosphides.
PCl5 in gaseous and liquid phases has a trigonal bipyramidal structure. The three equatorial — Cl bonds are equivalent, while the two axial bonds are longer than equatorial bonds. In Aid state, PCl5 exists as an ionic solid [PCI4]+ [PCl6]– in which cation is tetrahedral and the anion is octahedral.
Oxoacids of Phosphorus:
Phosphorus forms a number of oxoacids. Their basicity is equal to the number of P— OH bonds in the molecule. In oxoacids, phosphorus is tetrahedrally surrounded by other atoms. All these oxoacids contain atleast one P=Oand one P— OH bond. Hypophosphorous acid (H3P02) is a good reducing agent.
Some Important Reactions:
(i) For Ammonia (NH3):
AgCl (s) + 2NH3 (aq) –> [Ag(NH3)2] Cl (aq)
Cu2+ (aq) + 4NH3 (aq) ↔[Cu(NH3)4]2+ (aq) (Deep blue)
NH3 + Na0Cl—> NaNH2 + HCIO
NH3 (g) + H2O (I) ↔ NH4 (aq) + OH– (aq)
(ii) For Nitric Acid (HNO3):
3Cu + 8HNO3 (dil.) —–> 3Cu(NO3)2 + 2NO + 4H20
Cu + 4HNO3 (conc.)–> Cu(NO3)2 + 2NO2 + 2H20
I2 + 10HNO3 —–> 2 HI03 + 10NO2 + 4H20
C + 4HNO3 —> CO2 + 2H20 + 4NO2
S8 + 48HNO3 —> 8H2SO4 + 48NO2 + 16H20
P4 + 20HNO3 –> 4H3PO4 + 20NO2 + 4H20
(iii) For Phosphorus (P):
P4 + 3NaOH + 3H20 —> PH3 + 3NaH2PO4
P4 + 502 —> P4010
Ca3P2 + 6H20 —> 3Ca(OH)2 + 2PH3
Ca3P2 + 6HCl —> 3CaCl2 + 2PH3
3CuSO4 + 2PH3 —> Cu3P2 + 3H2S04
3HgCl2 + 2PH3 —> Hg3P= + 6HCl
(iv) For Phosphorus Trichloride (PCl3):
P4 + 8S0Cl2 –> 4PCl3 + 4S02 + 2S2Cl2
PCl3 + 3H20 —› H3P03 + 3HCl
3CH3COOH + PCl3 –> 3CH3COCl + H3PO4
(v) For Phosphorus Pentachloride (PCl5):
P4 + 10 SO2Cl2 —> 4PCl5 + 10 SO2
PCl5 + H2O —> POCl3 + 2HCI
POCl3 + 3H20 –> H3PO4 + 3HCl
PCl5 –∆–> PCl3 + Cl2
2Ag+ PCl5 –> 2AgCl + PCl3
Sn + 2PCl5 –> SnCl4 + 2 PCl3
(vi) For Hypophosphorous Acid (H3PO4):
4H3PO3 —> 3H3PO4 + PH3
4AgNO3 + 2H20 +H3P02 —> 4Ag + 4HNO3 + H3PO4
This article has tried to highlight all the important points of P Block Elements in the form of notes for class 12 students in order to understand the basic concepts of the chapter. The notes on P Block Elements have not only been prepared for class 12 but also for the different competitive exams such as iit jee, neet, etc.
Check Part 2 of the Chapter Here: Group 16 Elements – p block Elements
Check Part 3 of the Chapter Here: Group 17 Elements – p block Elements
Check Part 4 of the Chapter Here: Group 18 Elements – p block Elements